Supernova Remnants

When a star undergoes a supernova, its outer layers are ejected into space at velocities of thousands of kilometers per second (millions of miles per hour). This matter eventually runs up against the tenuous plasma that fills the near-vacuum between stars. When it does so, all of the matter heats to temperatures of tens of millions of degrees, causing it to glow against the dark background sky. Astronomer identify this hot plasma as supernova remnants, the remains of an exploded star.

With the immense energies of a supernova remnant, light is produced at all wavelengths, from radio to gamma-rays. The above of the remnant Cassiopeia A image is from the Chandra X-ray observatory. The X-ray light is produced by the 10 million degree material from the star, and the colors are coded to illustrate the locations of individual elements that used to be in the core of the star, such as iron, silicon, and oxygen.

Supernovae are important for the life-cycle of the universe, because they allow the heavy elements formed in the cores of stars to escape back into space. The elements that escape can then be incorporated into other stars, the planets that surround that, and in very rare cases, the life that emerges on the planets. This is the motivation for Carl Sagan's famous quote, "We are star stuff."

In addition, at the center of the remnant there lies a point of light, glowing white in this representation of X-rays. Astronomers believe that this object is the remains of the core of the star that exploded. When a star undergoes a supernova after having exhausted its fuel, it does not destroy itself completely. Although the outer layers are ejected into space, the core of the star continues to collapse.

For some stars, that collapse is halted when matter is compressed to the point that the nuclei of atoms run up against each other. At this point, gravity can be counteracted by the fundamental forces that determine the structure of an atom --- the Pauli exclusion principle in quantum mechanics, which states that no two particles want to share the same place, and the strong nuclear forces that are described by the theory of quantum chromodynamics. At these densities, it is not known how matter behaves. Early calculations predicted that protons and electrons would be forced to merge and form neutrons. Hence, the cores of these dead stars are referred to generically as neutron stars. However, more modern hypotheses predict other exotic reactions will occur, possibly breaking protons and neutrons into other particles, or even turning them into a soup of even more fundamental particles, quarks. Astrophysicists study neutron stars in part because they are the only place in the Universe in which we can gain insight into how matter at these densities behaves.

Neutron stars are found in several guises in our Galaxies. Most of the ones that we know of are pulsars, which are sometimes found at the centers of supernova remnants. A few are found nearby, glowing faintly as they radiate away the heat left over from the core of the star. A few have the strongest magnetic fields in the known Universe, and produce bursts of the highest energy light, gamma-rays, that can release ten thousand years' worth of the Sun's energy in a fraction of a second. Still others are identified because they are tearing apart stars that are orbiting to close. They show up as bright X-ray sources, because the matter they acquire gets heating to tens of millions of degrees as it falls toward the neutron star's surface. Neutron stars produce some of the strangest astronomical objects known.

Perhaps the only stranger objects are black holes. If the core of a star is too massive, sub-atomic forces will not be able to halt its collapse. The star will keep collapsing, until it cuts itself off from the rest of the Universe. This is a black hole.

The black holes that are produced when a star dies emit no light of their own. (Hawking radiation is extremely faint for a large black hole. Tiny black holes, if they exist, would be the only ones that would emit measurable Hawking radiation, and astrophysicists have not identified a sure way of producing tiny black holes in nature). Needless to say, this makes them very hard to find. The only stellar black holes that we know of are devouring stars that orbit too close by. Like their neutron star kin, these shine as X-ray sources.

Astronomers do not yet know what lies at the center of the Cassiopeia A supernova remnant above. Some have suggested that the fact that it glows brightly implies that it has a surface, and therefore that it must be a neutron star. Others have suggested that it could black hole, and that the glow is caused by matter that gets heated as it falls into the black hole. It remains a mystery.